[pstl] Remove our custom FindTBB CMake file

Summary:
The TBBConfig file installed by TBB 2019 Update 5 works properly, so we
don't need this workaround anymore.

Reviewers: rodgert, MikeDvorskiy

Subscribers: mgorny, jkorous, dexonsmith, libcxx-commits

Tags: #libc

Differential Revision: https://reviews.llvm.org/D60467

llvm-svn: 358196